Survey of 1,410 sites in Borneo and Wallacea finds forest cover an unreliable guide to bird diversity

On Buru, in Maluku, the forest remains relatively undisturbed. There the Buru racket-tail and the red lory live alongside the Buru flowerpecker ^[5]. The two parrots are both Psittaculidae. The flowerpecker is a passerine, joined to them only far back on the tree through the clade Psittacopasseres ^[6]. A species count scores that trio as three. Phylogenetic diversity adds up the branch lengths connecting a community, so three birds spanning that deep split hold more evolutionary history than three birds from one genus would ^[4].

Sangihe and Talaud supply the contrast. Sangihe's yellow-sided and gray-sided flowerpeckers are both Dicaeum, and both forage for small fruits in trees ^[8]. Talaud's elegant and gray imperial pigeons are both Ducula, relatively large-bodied birds that also eat fruit in trees ^[9]. Each pair sits close together on the tree. Mean nearest taxon distance measures how far each species is from its closest relative in the same community, and it is the yardstick designed to register pairs like these ^[4].

The researchers explain close relatedness through environmental filtering, in which local conditions decide which species survive ^[10]. They tie that filtering to dense forest, whose vegetation structure, light, food and foraging space suit certain groups of birds ^[10]. Their examples of close relatives, though, come from the two island groups where forest cover has fallen since 2010 ^[7]. Filtering could act in both settings, favouring forest specialists under dense cover and a narrow set of fruit eaters where cover is lost. The account was written by the researchers themselves for phys.org ^[14]. It does not report effect sizes, the survey method, or which of the three measures tracked forest cover across the 1,410 sites ^[1].

The forest measure is also local. A 250-metre radius encloses about 19.6 hectares ^[1], followed over 12 years ^[2]. Spread across seven islands, the sites average about 200 per island ^[3]. Cover at that scale describes the stand around a survey point. It does not say whether that point sits inside a large forest block or a small fragment.

Buton shows why cover is hard to read on its own. Its Sulawesi drongo, black sunbird and yellow-sided flowerpecker are all passerines, and their presence is partly linked to insect and nectar food ^[11]. The island's nearness to Sulawesi, across shallow straits, helped shape its bird fauna ^[12]. Island history and forest condition differ together across these seven islands. The thing this doesn't tell you is how much of Talaud's thin community reflects lost forest and how much reflects which lineages ever reached the island. A comparison of bird communities against forest conditions ^[3] cannot separate the two by itself.

I think the case for adding relatedness to forest assessments holds on the logic of the metrics alone, since a tally cannot tell a parrot-and-flowerpecker forest from a two-flowerpecker one. How large that difference is across real sites is the result still to come. The researchers wrote that conserving these communities and their diversity also means looking at their family tree ^[13].